The major goals of this project are to characterize the host's immune response to helminth infections and to relate the findings to the pathogenesis of clinical disease. Chronic filariasis and schistosomiasis are both characterized by cellular immune hyporesponsiveness to parasite antigens which may play a role in the persistence of the parasite within the host. The mechanisms involved in this hyporesponsiveness include both serum inhibitory factors and mononuclear suppressor cell populations. Profound immunologic hyperresponsiveness of the immediate type (IgE) characterizes the asthma-like tropical eosinophilia syndrome of human filariasis. Here the most important antigens (as determined by basophil histamine release experiments) appear to be those derived from microfilaria. In acute schistosomiasis not only is there marked cellular and humoral immune activity but also the presence of large numbers of circulating immune complexes which may be important in the syndrome's pathogenesis.